Typically, speech coding and audio (e.g., for music) coding at low bit-rates are approached differently. The speech coding is based on a speech production model with hybrid model and waveform based coding of an input signal. The speech production model parameters are quantized in a time domain. On the other hand, the audio coding utilizes transform coding in which the coding gain is achieved in the transform itself and in perceptual masking of transform coefficients before quantization.
Combining the model based time domain speech codec and transform based time-frequency domain codec has been a difficult task. There are no examples of successful algorithms achieving this goal without extensive delay in the algorithm to handle the transient from the time domain quantization to the transform coding.